The epithelial surfaces of the body serve as a barrier to antigenic material. However, those surfaces are by no means impenetrable. The mucosal immune system provides the next major line of defense against a majority of human pathogens. The mucosal immune system includes gut-associated lymphoid tissue (GALT), bronchus-associated lymphoid tissue, the salivary glands, the conjunctiva, the mammary gland, parts of the urogenital tract, and the middle ear.
GALT consists of two types of lymphoid aggregates. The first is referred to as Peyer's patches and the second consists of isolated lymphoid follicles. Peyer's patches have a defined micro-structure including a central B cell dependent follicle and T cell dependent regions adjacent to the follicle. The lymphocytes in Peyer's patches are heterogeneous, including B cells which express IgM, IgG, IgA, and IgE and various regulatory and cytotoxic T cells. Peyer's patches also contain specialized macrophages. The Peyer's patches are covered by M cells, which are specialized lympho-epithelium cells.
In GALT, ingested antigens produce a local immune response. The antigens are taken up by the M cells, which deliver the antigen to the underlying lymphocytes in the tissue. This results in the production of IgA at various secretory effector sites following the migration of activated lymphocytes through the efferent, lymphatic and circulatory system.
The absorption of antigens by the Peyer's patches can induce a systemic immune response if the antigen is taken up by macrophages in the Peyer's patches. Macrophages induce a systemic response by processing antigens and presenting them to lymphocytes. The lymphocytes then become activated and cause the production of systemic antibodies specific to the antigens.
Childers et al. (Oral Microbiol. Immunol. 1994:9:146-153) reported that lyophilized liposomes containing S. mutans antigen can be administered orally to human patients and will be absorbed by GALT to elicit a local immune response. No systemic response was observed however.
Traditionally, to obtain a systemic immune response by oral administration of an antigen, it was required that the antigen be associated with an adjuvant. The presence of the adjuvant permits the antigen/adjuvant combination to be recognized by the CD4 cells, which send signals to B cells to produce antibodies, and by the cytotoxic lymphocytes, which kill the infecting organism in affected host cells. Without the presence of the adjuvant, the CD4 cells and cytotoxic lymphocytes ignore the free antigen.
Typical adjuvants include alum, Freund's adjuvant, incomplete Freund's adjuvant and indotoxin. These adjuvants typically induce an inflammatory response. Other typical adjuvants are immuno stimulating complexes (iscoms) that contain Quil A. These adjuvants typically cause clumping of antigens.
A need therefore exists to induce a systemic immune response by oral administration without the presence of an adjuvant and without inducing the above-described adjuvant effects, but instead by uptake by the macrophages in the Peyer's patches.